Client: Knight Group
Location: Great Wakering, Essex
As part of Phase 2 Taylor Wimpey housing development at Great Wakering temporary wellpoint dewatering of shallow River Terrace Deposits is required to enable construction of deep foul and storm new pipeline sections where the excavation level is below groundwater level. It was envisaged that a nominal 400 lin.m of wellpoint dewatering will be required along the deeper drainage sections. Maximum abstraction will be from 2 x 90m wellpoint systems at any one time, with maximum abstraction from both system at 9 lts/sec (778 m3/day). Abstracted groundwater would flow via a v-notch settlement tank, with gravity drainage thereafter to a drainage ditch to the south of the site, until the Attenuation Tank system is in place.
As the dewatering abstraction rate and time scale would fall within UK Environmental Regulations affecting Construction Dewatering an EA application for groundwater abstraction and recharge was required. A central part of the application process is a Hydrogeological Impact Assessment (HIA) to demonstrate what effect the dewatering operation will have and if it will contribute to any regional water resource issues.
Stuart Wells undertook the HIA which includes various elements from where the site fits in with any regional water resource status, a conceptual model, assessment of potential water features that are susceptible to flow impacts together with the likely impacts to these water features, assessment of drawdown impact and mitigation measures to any drawdown impacts with water quality and monitoring strategy.
The HIA highlighted adjacent Fishing Lakes and that water levels within the lakes where likely to be impacted by the dewatering operation. As a mitigation measure a T-junction with control gate valves would be established along the discharge line to allow all or part of the discharge water to be diverted back into the Fishing Lake. A monitoring strategy for lake water levels was established. EA consent was applied for and subsequentially approved, with the dewatering successfully undertaken.